Smashing Magazine has put together an article comparing sveral of the popular PHP IDEs on a list of about twenty different features:

Everyone wants to be more productive, make fewer mistakes and write good code. Of course, that all depends on you, but in most cases integrated development environments (IDEs) can help you achieve those goals more easily. Unfortunately, choosing the right IDE is very difficult because a lot needs to be considered. [...] In this post, we’ll take a close look at the most popular PHP IDEs, exploring their functions, comparing them in a table and drawing some conclusions.

The article is less about finding the "best of the best" our of the reviewed IDEs and more about helping you figure out which one would be the right fit for you. They compare them in the features they offer - syntax highlighting, navigation, debugging, versioning and more. There's even a handy chart with all of these laid out for each editor they reviewed (PDT 1, PDT 2.0, Zend Studio 6 & Netbeans 6.5).